Mountains of Obfuscation

You have a couple hundred files across several nested folders that contains within them 10000+ obfuscated random and unique user names. You also have a translation file of all of the obfuscated names and new ones you ned to have them replaced with. Some files have all 10000+ obfuscated names in them. Some have none or just a couple.

How would you update all of the files?

Obviously this isn’t a crazy hard problem; there are many ways to solve it. I know how I solved it. I am curious as how YOU would solve it.